An object may be made effectively invisible at least over some frequency range. This has been termed a “cloak of invisibility”; the invisibility sought may be partial at a specific frequency, or over a band of frequencies, so the term “cloak of invisibility” or “cloak” may take on a variety of meanings. The cloak may be designed to decrease scattering (particularly “backscattering”) from an object contained within, while at the same time reducing the shadow cast by the object, so that the combination of the cloak and the object contained therein have a resemblance to free space. When the phrase “cloaking,” “cloak of invisibility,” or the like, is used herein, the effect is generally acknowledged to be imperfect, and the object may appear in a distorted or attenuated form, or the background behind the object by the object may be distorted or partially obscured.
As will be understood by a person of skill in the art a “frequency” and a “wavelength” are inversely related by the speed of light in vacuo, and either term would be understood when describing an electromagnetic signal.
In some aspects, the cloak has a superficial similarity to “stealth” technology where the objective is to make the object as invisible as possible in the reflection or backscattering direction. One means of doing this is to match the impedance of the stealth material to that of the electromagnetic wave at the boundary, but where the material is strongly attenuating to the electromagnetic waves, so that the energy backscattered from the object within the stealth material is strongly attenuated on reflection, and there is minimal electromagnetic reflection at the boundary within the design frequency range. This is typically used in evading radar detection in military applications. Shadowing may not be a consideration in stealth technology. Shadowing may be understood as the effect of the object in blocking the observation of anything behind the object, for example the background, where the object is disposed between the observer and the background. A perfect cloak would result in no shadowing.
The materials used for the cloak may have properties where, generally, the permeability and permittivity tensors are anisotropic and where the magnitudes of the permeability and permittivity are less than one, so that the phase velocity of the electromagnetic energy being bent around the cloaking region is greater than that of the group velocity.
Materials having such properties have not been discovered as natural substances, but have been produced as artificial, man-made composite materials, where the permittivity and permeability of the bulk material are less than unity, and may be negative. They are often called “metamaterials” an extension of the concept of artificial dielectrics, that were first designed in the 1940s for microwave frequencies. Such materials typically consist of periodic geometric structures of a guest material embedded in a host material.
Analogous to the circumstance where homogeneous dielectrics owe their properties to the nanometer-scale structure of atoms, metamaterials may derive their properties from the sub-wavelength structure of its component materials. At wavelengths much longer than the unit-cell size of the material, the structure can be represented by effective electromagnetic parameters that are also used describe homogeneous dielectrics, such as an electric permittivity and a refractive index.
Cloaking has been experimentally demonstrated over a narrow band of microwave frequencies by achieved by varying the dimensions of a series of split ring resonators (SRRs) to yield a desired gradient of permeability in the radial direction.